In 3GPP (3rd Generation Partnership Project), which is a project aiming to standardize a mobile communication system, specifications are being designed to enhance LTE (Long Term Evolution) in order to comply with the rapidly increasing traffic demands.
Further, in order to respond to a rapidly increasing traffic demand, focus is now paid not only to communication using a frequency band in which a license is required (licensed band) but also to communication using a frequency band in which no license is required (unlicensed band/unlicensed spectrum).
Here, according to a law (for example, Wireless Telegraphy Act in Japan), if a radio signal is transmitted by using the unlicensed band, then it is required to execute a CCA (Clear Channel Assessment) before the radio signal is transmitted. Specifically, a base station measures interference power in the unlicensed band. If the measurement result is good (specifically, if the interference power is low), it is possible to transmit a radio signal in the unlicensed band.